Switch from antsibull to antsibull-docs. (#438)

tests/sanity/extra/extra-docs.json

@@ -5,6 +5,6 @@
     ],
     "output": "path-line-column-message",
     "requirements": [
-        "antsibull"
+        "antsibull-docs"
     ]
 }

tests/sanity/extra/extra-docs.py

@@ -1,7 +1,7 @@
 #!/usr/bin/env python
 # Copyright (c) Ansible Project
 # GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
-"""Check extra collection docs with antsibull-lint."""
+"""Check extra collection docs with antsibull-docs."""
 from __future__ import (absolute_import, division, print_function)
 __metaclass__ = type